Magnetrons are compact narrowband high power microwave sources primarily used to generate power at microwave frequencies. Magnetrons are used in applications, for example, radar systems, microwave oven devices, plasma screen apparatuses, plasma lighting, manufacturing such as medical device manufacturing, and linear accelerator applications to name a few.
One of the first magnetrons was the two-pole magnetron, also known as a split-anode magnetron. A major problem with the two-pole magnetron was low efficiency. As a result, the cavity magnetron was developed, which proved to be far more useful.
The basic construction of a cavity magnetron includes a cathode, anode, antenna, cavities and waveguide. The cathode is centrally located in a chamber. The anode surrounds the cathode and includes vanes that form cavities at a fixed radius from the cathode. The entire assembly is placed in a powerful magnetic field. The magnetic field is parallel to the axis of the cathode and is imposed by a permanent magnet or pair of Helmholtz coils. The magnetic field causes electrons emitted from the cathode to spiral outward in a circular path rather than moving directly to the anode. As the electrons sweep past the cavities, they induce a resonant, high-frequency radio field within the cavities. A portion of this field is extracted with a short antenna that is coupled to a waveguide. The antenna is a probe or loop that is connected to the anode and extends into the cavities. The antenna transmits the extracted high-frequency radio field, or RF energy, into the waveguide. The waveguide then directs the RF energy to the load. The load, for example, may be a cooking chamber in a microwave oven or a high-gain antenna in a radar system.
Different types of extraction of the radio field can be applied, such as radial extraction and axial extraction. The size of the cavities determine the resonant, high-frequency radio field, and thereby the frequency of emitted microwaves known as the radiation pattern.
The most compact narrowband high power microwave source is known as a relativistic magnetron with axial extraction of electromagnetic energy, also known as the magnetron with diffraction output (MDO). Its advantages compared to conventional magnetrons are its compactness, improved resistance to microwave breakdown, and the ability to use any mode as the operating mode. In addition, possible mode hopping is not dangerous. However, the radiation pattern of a MDO is more complicated than that of the conventional magnetron that can make it difficult for some applications. What is needed is a relativistic magnetron that creates simpler radiation patterns. The present invention satisfies this need.